This invention relates to a process for isomerization of alkylaromatics. The process involves contacting the alkylaromatics with a catalyst at isomerization conditions with water.
The xylenes, i.e., para-xylene, metaxylene and orthoxylene, are important intermediates which find wide and varied application in chemical synthesis. Para-xylene upon oxidation yields terephthalic acid which is used in the manufacture of synthetic textile fibers and resins. Metaxylene is used in the manufacture of plasticizers, azo dyes, wood preservers, etc. Orthoxylene is feedstock for phthalic anhydride production.
Xylene isomers from catalytic reforming or other sources generally do not match demand proportions as chemical intermediates, and further contain ethylbenzene which is difficult to separate or convert. Para-xylene in particular is a major chemical intermediate with rapidly growing demands, but amounts to only 20-25% of a typical C8 aromatics stream. Adjustment of isomer ratio to demand can be effected by combining xylene-recovery, such as adsorption for para-xylene recovery, with isomerization to yield an additional quantity of the desired isomer. Isomerization converts a non-equilibrium mixture of the xylene isomers which is lean in desired components to a mixture approaching equilibrium concentrations.
An increasingly close approach to equilibrium of C8 aromatic isomers in an isomerization process is associated with higher losses of C8 aromatics to other hydrocarbons. A close approach to equilibrium minimizes the amount of recycle to para-xylene recovery, and thus reduces the investment and operating costs of the complex. A lower loss of C8 arornatics reduces feedstock requiremnents, and increasesi the proportion of higher-value products. The performance of an isomerization process is determined principally by the interrelationship of conversion, C8 aromatic losses and catalyst stability. The operating temperature required to achieve a given conversion is an indicator of such performance, since higher temperatures generally result in higher losses to byproducts and more rapid catalyst deactivation.
Catalysts for the upgrading of C8 aromatics to improve isomer distribution ordinarily are characterized by the manner of processing ethylbenzene associated with the xylene isomers. Ethylbenzene is not easily isomerized to xylenes, but it normally must be reacted because separation from the xylenes by superfractionation or adsorption is very expensive. Modern approaches to C8 aromatics isomerization include reaction of the ethylbenzene in the presence of a solid acid catalyst with a hydrogenation-dehydrogenation function to effect hydrogenation to a naphthene intermediate followed by dehydrogenation to form a xylene mixture. Another approach is to convert ethylbenzene via dealkylation to form principally benzene while isomerizing xylenes to a near-equilibrium mixture. The former approach enhances xylene yield by forming xylenes from ethylbenzene, but the latter approach commonly effects higher ethylbenzene conversion and thus lowers the quantity of recycle to the para-xylene recovery unit with a concomitant reduction in processing cost. The latter approach also yields a high-quality benzene product.
U.S. Pat. No. 4,899,012 (Sachtler et al.), teaches the process of ethylbenzene dealkylation and xylene isomerization using the pentasil group (MFI, MEL, MTW, MTT and FER) of zeolitic aluminosilicates. U.S. Pat. No. 4,740,650 (Pellet et al.) teaches isomerization using at least one non-zeolitic molecular sieve which preferably is a silicoalurninophosphate. U.S. Pat. No. 5,276,236 (Patton et al.) discloses a MgAPSO non-zeolitic molecular sieve and its use in the process of alkylaromatic isomerization. U.S. Pat. No. 5,898,090 (Hammerman et al.) teaches isomerization of a mixture of xylenes and ethylbenzene using an SM-3 silicoaluminophosphate molecular sieve.
In the isomerization art relating to combination schemes, in which the isomerization process is disclosed in the context of product recovery technology, there is a strong implication that the isomeriationfeed will be dry. For example. U.S. Pat. Nos. 3,856,872 (Morrison), 4,218,573 (Tabak et al.) and Re 31,782 (Olson et al.) disclose various flow schemes including a crystallizer providing feedstock to the isomerization process. It is known in the art that a crystallizer recovering para-xylene must be a dry operation, and the raffinate from the crystallizer to the isomerization process thus will be dry. U.S. Pat. No. 4,584,423 (Nacamuli et al.) discloses prefractionation and crystallization prior to isomerization, and both steps would remove water from the isomerization feed.
U.S. Pat. No. 4,300,013 (Whittam), on the other hand, discloses the addition of large quantities of water (0.05-1.0 wt. %) in an alkylbenzene isomerization process based on zeolite FU-1. U.S. Pat. No. 4,723,050 (Butler et al.) also discloses the supply of large quantities of water (0.1-10 wt. %) to an isomerization process based on the idea that steam is believed to reduce catalyst coking due to sulfur.
U.K. Patent Specification 1,255,459 (Hart et al.) discloses steam addition in an amount ranging from 100 to 1500 ppm for an xylene isomerization process. Hart et al. discloses ethylbenzene isomerization to xylenes through naphthenes using a catalyst based on an acidic refractory oxide containing a mixture of silica and alumina. Also disclosed is the use of zeolitic aluminosilicates such as faujasite (FAU).
German Patent DD-219,183 (Doms et al.) discloses a mordenite (MOR) based catalyst contacted with NH3, where the selectivity of ethylbenzene isomerization is improved by the addition of 500 to 800 ppm water in the initial reaction period up to a running time of 300 hours followed by reduction of the water content to the range of 10 to 50 ppm.
Other disclosures of the art include U.S. Pat. No. 3,381,048 (Lovell et al.) and U.S. Pat. No. 3,898,297 (Sampson et al.) both of which are based on amorphous catalyst systems. Lovell et al. disclose for a xylene isomerization process based on a platinum-alumina-halogen catalyst system that process conditions be controlled so that water content is dry on the order of 20 to 200 wt-ppm during reaction and wet on the order of 0.3 to 2.0 wt % during regeneration. Sampson et al. disclose an alkyl benzene isomerization process based on fluorided variations of amorphous aluminas or silica/aluminas optionally containing alkali or alkaline earth metals in the presence of steam in the range of 0.005 to 1.0 wt %.
U.S. Pat. No. 4,431,857 (Feinstein) discloses the use of crystalline borosilicate (AMS-1B) impregnated with molybdenums and the use of water addition to favor A10 formation via a disproportionation mechanism.
U.S. Pat. No. 5,773,679 (Beck et al.) discloses the co-feeding of water during the initial operation of selectivated ZSM type zeolites for hydrocarbon conversion, and the use of silicon treated ZSM-5 for toluene disproportionation in particular. The effect of this zeolite treatment is to increase para-selectivity of the catalyst by decreasing the yield of xylenes, with the total xylenes decreasing a greater amount than the para-isomer such that the relative ratio of para-xylene increases, and with the resultant balance showing increased yield of benzene. No effect on ethyl-benzene conversion is shown.
Nothing in the prior art suggests the use of trace quantities of water injection during alkylaromatic isomerization reaction conditions with non-zeolitic molecular sieve containing catalysts in order to effect improved or reactivated catalyst performance while maintaining the presence of hydrocarbon material in the system. Further, the need to improve the use selected pentasil zeolitic aluminosilicate containing catalysts that have not been silicon selectivated and where the water injection may be provided continuously or intermittently to promote favorable ethylbenzene conversion also is addressed by the present invention.
In summary, it is an object of the present invention to provide an improved process for the isomerization of xylenes and conversion of ethylbenzene.
This invention is based on the discovery that injection of trace amounts of water or a water producing compound into the reactor zone of a C8 aromatics isomerization process results in a surprising improvement in ethylbenzene conversion and para-xylene yield at given reactor temperatures, along with improved catalyst stability.
A broad embodiment of the present invention is a process for the upgrading of a nonequilibrium C8 aromatics feed mixture wherein water or a water forming compound is supplied continuously or intermittently to a reaction zone containing a catalyst comprising either a non-zeolitic molecular sieve or a pentasil zeolitic aluminosilicate in order to increase xylenes isomerization and ethylbenzene conversion at a given temperature.
In a specific embodiment, the catalyst further comprises at least one platinum-group metal component, and an inorganic-oxide binder. The equivalent water provided to the reaction zone corresponds to about 75 to about 750 mass ppm, or preferably corresponds to about 100 to about 500 mass ppm.
These as well as other objects and embodiments will become apparent upon a reading of the following detailed description of the invention.